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Beginning in 2007, most of the United States begins Daylight Saving Time at 2:00 a.m. on the second Sunday in March and reverts to standard time on the first Sunday in November. In the U.S., each time zone switches at a different time.

So this year on March 11th 2007, when you get up in the morning it will be an hour later than what you think it is.

In the European Union, and Australia Summer Time begins and ends at 1:00 a.m. Universal Time (Greenwich Mean Time). It begins the last Sunday in March and ends the last Sunday in October. In the EU, all time zones change at the same moment.

In the U.S., clocks change at 2:00 a.m. local time. In spring, clocks spring forward from 1:59 a.m. to 3:00 a.m.; in fall, clocks fall back from 1:59 a.m. to 1:00 a.m. In the EU, clocks change at 1:00 a.m. Universal Time. In spring, clocks spring forward from 12:59 a.m. to 2:00 a.m.; in fall, clocks fall back from 1:59 a.m. to 1:00 a.m.

In the U.S., restaurants and bars have various closing policies. In many states, liquor cannot be served after 2:00 a.m. But at 2:00 a.m. in the fall, the time switches back one hour. So, can they serve alcohol for that additional hour in October? The official answer is that the bars do not stop serving liquor at 2:00 a.m., but actually at 1:59 a.m. So, they have already stopped serving when the time changes from Daylight Saving Time into Standard Time. In practice, however, many establishments stay open an extra hour in the fall.

In the U.S., 2:00 a.m. was originally chosen as the changeover time because it was practical and minimized disruption. Most people were at home and this was the time when the fewest trains were running. It is late enough to minimally affect bars and restaurants, and it prevents the day from switching to yesterday, which would be confusing. It is early enough that the entire continental U.S. switches by daybreak, and the changeover occurs before most early shift workers and early churchgoers are affected.

In the US, cellphones update automatically when there is an incoming event…either an SMS or a call. Computers should also update if your computer has the latest Windows Updates. Make sure that you do, otherwise your Outlook Calendar events are going to be all awry.

More on DST here.